How much N will soil really acquire from a legume cover crop? Let’s take it from the point of a freshly killed, annual legume, cut down in its prime at mid-bloom.The management and climatic events following the death of that legume will greatly affect the amount and timing of N release from the legume to the soil. Most soil bacteria will feast on and rapidly decompose green manures such as annual legumes, which contain many simple sugars and proteins as energy sources. Soil bacteria love to party and when there is lots to eat, they do something that no party guest you’ve ever invited can do—they reproduce themselves, rapidly and repeatedly,doubling their population in as little as seven days under field conditions (306). Even a relatively inactive soil can come to life quickly with addition of a delectable green manure. The result can be a very rapid and large release of nitrate into the soil within a week of the green manure’s demise. This N release is more rapid when covers are plowed down than when left on the surface. As much as 140 lb. N/A has been measured 7 to 10 days after plowdown of hairy vetch (363).Green manures that are less proteinrich (N-rich) will take longer to release N. Those that are old and fibrous or woody are generally left for hard-working but somewhat sluggish fungi to convert slowly to humus over the years, gradually releasing small amounts of nutrients. Other factors contribute significantly to how quickly a green manure releases its N.Weather has a huge influence.The soil organisms responsible for decomposition work best at warm temperatures and are less energetic during cool spring months. Soil moisture also has a dramatic effect. Research shows that soil microbial activity peaks when 60 percent of the soil pores are filled with water, and declines significantly when moisture levels are higher or lower (244).This 60 percent water-filled pore space roughly corresponds to field capacity, or the amount of water left in the soil when it is allowed to drain for 24 hours after
a good soaking rain. Microbes are sensitive to soil chemistry as well. Most soil bacteria need a pH of between 6 and 8 to perform at peak; fungi (the slow decomposers) are still active at very low pH.Soil microorganisms also need most of the same nutrients that plants require, so low-fertility soils support smaller populations of primary decomposers, compared with high-fertility soils.Don’t expect N-release rates or fertilizer replacement values for a given cover crop to be identical in fields of different fertility. Many of these environmental factors are out of your direct control in the near term.Management factors such as fertilization, liming and tillage, however, also influence production and availability of legume N.

So while know that cover crop legumes produce nitrogen we also know that the nitrogen will be released at different times and in different amounts according to the circumstances in individual fields and with different management practices.

In 2010 many producers across the Midwest shared that they could see a “big difference” in their corn yield where they had a legumes as a cover crop. While we cannot easily (or maybe not totally) quantify the pounds of N available for the next crop we can count on some level of improved performance. Can we reduce our inputs? Maybe. Can we always get higher yields? No. Can we anticipate an improved yield? Probably. Some folks will tell you “yes” to those questions! Some others will have a different experience.

So what do you do? Plan for success, scout your corn, use a pop-up and/or starter with nitrogen…and scout…oh, yeah, I guess I already said that!

Puny, they just look puny this year! Those Austrian Winter Peas are supposed to be 30″ tall like 2009…well, with less than 2″ of rain in NW Ohio since mid-July, I guess they look pretty good…but still puny. THAT is why I take a shovel with me…I want to see what they are really doing IN and TO the soil. Take a look at this video to see what I found. Thanks to Greg Fretti from CISCO for helping video this segment so I could dig.

“What should we recommend for our producers to plant as a cover crop after the wheat comes off?”

After running through a number of scenarios I determined that I’d better make sure I don’t confuse folks with too many ideas. So, here are a few scenarios that I hope you find useful.

If you want to scavenge Nitrogen and other nutrients, then a good choice would be Oats and Radishes mixed together. This mix generally should be planted at 1 1/2 bu Oats and 2-5# of Oilseed radish/acre. This mix also requires manure or around 40# N applied for it to be the ideal mix for this situation. This mixture will die in the winter and provide excellent winter annual weed control. Best planting time is Mid August…so you have plenty of time to get weeds killed and manure applied.

Sudangrass and Sorghum-Sudangrass are excellent crops after wheat if you need animal feed.

If you have livestock to feed, other great nutrient scavengers to plant after wheat are Sudangrass, Pearl Millet, or Sorghum-Sudangrass. These summer annual grasses require 100+# of N or equivalent manure. Tonnage of 4 1/2 tons Dry matter/acre have been common in northern Indiana following wheat the past three years. These grasses are excellent soil builders as well. Mycorrhizae fungus find the roots of these plants a very good place to multiply and build soil structure.

Oats and Radish Mixture following wheat. Hog manure was nitrogen source.When properly inoculated, Austrian Winter Peas are prolific producers of nodules (and nitrogen).Oilseed Radishes are "nutrient sorage facilities"! These big tubers take in significant amounts of nitrogen and other nutrients.

If you want to produce nitrogen and also scavenge nutrients as well, then an excellent choice will be Austrian Winter Peas mixed with Oilseed Radish . This mixture works very well without the addition of additional nitrogen. Count on the peas to deliver 60-120# of N and the oilseed radishes to keep most all of it in the top soil root zone. This mixture does a great job of controlling weeds. The radishes will die in the winter and the peas will “probably” die too…depending on your location, planting time of the mixture, and snow cover. If they overwinter they are easily killed in the spring.

If you are looking mainly fornitrogen production with a species that will winterkill, then choose cowpeas. The past few years cowpeas have been very expensive to plant. Be sure to check pricing and also be sure to inoculate the seed at planting time. Sow 60-100#/acre of cowpeas.

If you want the nitrogen production with a crop that will overwinter, then crimson clover is a top choice. Excellent new varieties have come on the market the past few years. Be sure to inoculate this seed as well. Sow 20-25#/acre of crimson clover.

Crimson clover planted after wheat is an excellent nitrogen producer.

There are several other options but these will certainly give you a good start in making selections.